1. Field of the Invention
Embodiments of the present invention generally relate to optical signal processing techniques and, more particularly, to optical signal processing of interrogation signals received from time-division multiplexed (TDM) interferometers.
2. Description of the Related Art
Marine seismic exploration surveys for the exploration and monitoring of hydrocarbon producing zones and reservoirs utilize seismic cables having sensor arrays, i.e., a plurality of sensor stations interconnected by sections of cable. The cable arrays may include a large number of sensor stations (e.g., several hundreds or thousands) and may be buried in a predetermined pattern on the ocean floor. Optical sensors may be particularly well-suited for ocean bottom seismic (OBS) applications, due to their robust nature, lack of sensitive electronics, and potential for lightweight sensors and cable assemblies that are relatively inexpensive to install. An optical sensor station may include a number of interferometric sensors, such as optical hydrophones, accelerometers along multiple axes, and/or geophones.
An interferometric sensor system may comprise a transmitter unit that produces an interrogation signal for the interferometric sensors, a sensor network, and a receiver unit that detects the signals from the sensor network. The sensor network may comprise several optical pathways from its input to its output, and some pairs of optical pathways form sensor interferometers. These optical pathways are called sensor pathways. Each sensor interferometer comprises a sensor and lead paths, the parts of the two sensor pathways that are not common define the sensor, while the common parts define the lead paths. In a fiber optic sensor network the lead paths are called lead fibers. The portion of the lead paths between the transmitter unit and a sensor is called the down-lead path and the portion of the lead paths between a sensor and the receiver unit is called the up-lead path. The portion of the lead paths that are common to both the down-lead path and the up-lead path is called the common lead path, or common lead fiber for a fiber optic sensor network. The sensors interferometer can be Michelson interferometers, Mach-Zehnder interferometers or Fabry-Perot interferometers. The sensor network can be a number of topologies, including a star network, a ladder network, a transmissive serial array, a serial Michelson array or an inline Fabry-Perot sensor array. The different paths through the sensor network may typically be formed by optical waveguides and splitters like optical fibers, optical splitters, circulators, and other waveguide coupled components, or free space optical paths, mirrors, beam splitters and other bulk components. The time delay difference τs between the two sensor pathways is called the imbalance of that sensor, which is typically equal for all sensors.
The sensor phase, which is the phase delay difference between the two sensor pathways, can be made sensitive to some physical property that one wants to measure. Thus, information about the physical property can be found by using time-division multiplexing (TDM), for example, and extracting the phase of the interference (optically detected as a fringe signal) between the interrogation signal that has propagated the two sensor pathways. TDM of an interferometric sensor network is a form of pulsed interrogation that is achieved by producing light pulses within the transmission unit and transmitting the pulses into the sensor network in one or more pulse transmission time intervals. Signal processing means may periodically measure the phase of the fringe signal to thereby measure changes in the physical property.
A fundamental difference between an interferometric sensor and most electronic sensors is that the phase of a real sinusoidal signal (the fringe signal) is extracted. Not only should the bandwidth of the sensor signal be taken into consideration, but the bandwidth of the fringe signal should also be considered for an interferometric sensor. The bandwidth of the sensor signal may be on the order of a few hundred Hz, for example. For small sensor signals less than 1 rad, the bandwidth of the fringe signal is approximately equal to the bandwidth of the sensor signal. However, for sensor signals larger than 2π, the bandwidth of the fringe signal is equal to the derivative of the phase, i.e., the slew rate of the sensor signal. For interferometric applications with a large dynamic range required for demodulation, such as OBS where the strong first brake from the gun array and weak reflections from the ground layers are both demodulated, conventional demodulation techniques may suggest a relatively large bandwidth to demodulate the fringe signal, even though the bandwidth of the sensor signal may be orders of magnitude lower. Thus, conventional demodulation techniques may require considerable computation resources and may even be impossible to implement for some signal processing systems.
One solution to the problem of processing interferometric signals having high fringe rates is described in U.S. Pat. No. 6,606,186 to Maas, entitled “Dynamic Fiber Optic Sensor Signal Processing Scheme,” filed Jul. 25, 2001. In an effort to optimize bandwidth utilization of demodulators in a system for seismic signal processing, Mass teaches the use of two demodulators, wherein the bandwidth of one demodulator is greater than the bandwidth of the other demodulator, and an optical switch having a switch control input electronically coupled to the output of a fringe rate detector. The optical signals are switched among the demodulators depending on the fringe rates such that signals are only routed to the high performance demodulator (i.e., the demodulator with the higher bandwidth capable of processing signals having high fringe rates) when necessary. In one aspect of the invention, optical signals are routed to a first demodulator, the fringe rate for each of the optical signals is sensed, and a signal line having fringe rates exceeding a threshold fringe rate is detected. The method further comprises rerouting the signal lines to a second demodulator, detecting a condition of the signal line, and rerouting the signal line from the second demodulator back to the first demodulator when the signal line has fringe rates below the threshold fringe rate input for the first demodulator. However, this technique requires a second high performance demodulator, an optical switch for routing signals between the two demodulators, and logic to control the optical switch.
Accordingly, there is a need for optical signal processing techniques, apparatus, and systems that effectively reduce the bandwidth of the fringe signal in interferometric sensors such that a large dynamic range in the demodulated sensor phase is achieved.